This research will seek to explore the development of complex perceptual abilities during infancy, including the ability to discriminate change of a sound's location and what cues are important in localization. Very little information now exists on the development of auditory localization during early infancy, yet this ability is very basic to hearing and experiencing sounds in the environment. The proposed research would investigate infants' ability to discriminate directionality of sounds in a free field acoustic environment using sounds from one loudspeaker, or from two loudspeakers located to the left and right of the subject. The latter sound stimulus would consist of one sound leading the other in time as to give the perception of a single sound from a single source located on the side of the leading sound. This phenomenon, known as the precedence effect, appears to require cortical processing in order for the subject to perceive the two sounds as one in this situation, and is therefore a more difficult discrimination than direction of a single source sound. Auditory discriminations of varying difficulty will be tested, using heart rate change and behavioral measures such as head turning and eye movements. The detection of simple shifts in location as well as use of localization cues in operant conditioning tasks will be assessed. Duration, frequency, amount of time delay between sound onsets, and type of sound (speech vs. nonspeech) will be assessed as factors important to localization. Infants from birth to 6 months will be tested, as well as preschool children and adults.